I asked this same question myself a month ago. Now after some practical experience with several different snakes, I'm gonna answer this way:
There are pros and cons to feeding in the viv. The three major objections are:
1. Ingestion of substrate. Even if you feed on a paper plate or some other item, your snake may drag his dinner into the substrate and accidentally swallow something unwanted.
2. Failure to find food. Even in a viv as small as a 10 gal tank, some hatchlings may not find their prey until it's cold and unappetizing.
3. If your snake starts to equate the opening of his viv with dinnertime, he might start mistaking your pinkie for his pinkie.
On the other hand, there are some issues with feeding in a separate container:
1. A shy snake may be too stressed to eat after being scooped out of their comfy home and slapped into a cold, bare container.
2. Many a snake has made good his escape while be transferred from the viv to container or back. (This is rare, but has happened to the most experienced of us. See Dean's recent post on his King)
3. If you have a lot of snakes to feed, it's a pain in the butt washing your hands between handling snakes and washing containers between snakes and keeping track of how long each snake has been with its intended meal.
Personally,I feed five of my six snakes in their vivs. The sixth one, I acquired as a yearling and he had been trained to eat outside his viv. I feed him in a card board box. I put his mousie in the box, put him in the box, close the box, and put it in a dark quiet place for 30 minutes.
My recommendation is do what works for you. The more comfortable you are with the routine, the more likely you will enjoy your snakes. And that's what's important in this. Good Luck!